Pharmaceutical compositions comprising (3-(1-(1H-imidazol-4-yl)ethyl)-2-methylphenyl)methanol

ABSTRACT

Disclosed herein is a pharmaceutical composition comprising (3-(1-(1H-imidazol-4-yl)ethyl)-2-methylphenyl)methanol and methods of using the composition to treat chronic pain.

CROSS REFERENCE

This application is a continuation of, and claims priority to, U.S.patent application Ser. No. 13/147,760, filed Aug. 25, 2011, which is anational stage application filed under 35 U.S.C. §371 of InternationalApplication No. PCT/US10/24111, filed on Feb. 12, 2010, which claimspriority to and the benefit of U.S. Provisional Patent Application No.61/152,494 filed on Feb. 13, 2009, the disclosure of each of which ishereby incorporated by reference in its entirety.

Disclosed herein are compounds having the structure

The structure depicted above includes the following tautomeric forms:

Compound I is (3-(1-(1H-imidazol-4-yl)ethyl)-2-methylphenyl)methanol. Itoccurs as two enantiomers:

Compound II is(R)-(3-(1-(1H-imidazol-4-yl)ethyl)-2-methylphenyl)methanol;

Compound III is(S)-(3-(1-(1H-imidazol-4-yl)ethyl)-2-methylphenyl)methanol. Thesecompounds are formed by the in vivo hydroxylation of medetomidine(5-(1-(2,3-dimethylphenyl)ethyl)-1H-imidazole):

Medetomidine is well known in the art. The hydrochloride salt of the (S)enantiomer of medetomidine is sold in the United States under the brandname Precedex® for the sedation of initially intubated and mechanicallyventilated patients during treatment in an intensive care setting. Theracemic mixture (medetomidine) is sold in the United States for use inthe sedation of animals.

Metabolites of medetomidine and dexmedetomidine are known, and aredescribed in I. Stoilov, et al., Synthesis of detomidine andmedetomidine metabolites: 1,2,3-trisubstituted arenes with4′(5′)-imidazolylmethyl groups, J. Heterocyclic Chem. 30:6, 1645-51(1993); P. V. Kavanagh et al., Synthesis of possible metabolites ofmedetomidine {1-(2,3-dimethylphenyl)-1-[imidazol-4(5)-yl]ethane}, J.Chem. Res. Synopses 4, 152-3 (1993); J. S. Salonen and M. Eloranta,Biotransformation of medetomidine in the rat, Xenobiotica 20:5, 471-80(1990); Y-H Hui et al., Analytical method development for thesimultaneous quantitation of dexmedetomidine and three potentialmetabolites in plasma, J. Chromatog., 762, 281-291 (1997).

The inventors have discovered that, whereas medetomidine issedating—indeed, whereas the compounds is marketed for its sedativeproperties—the compounds of the invention may be used to treat chronicpain without sedation.

Synthesis

Compounds of Formula I may be synthesized as follows.

Procedure:

To a 1 L, one-necked flask equipped with a magnetic stir bar and refluxcondenser, was added 3-bromo-2-methylbenzoic acid 1 (200 g, 0.93 mol)and thionyl chloride (202 mL, 2.79 mol). The reactor was attached toaqueous NaOH solution scrubber (the amount of NaOH was not listed above)and heated to 76° C. for 90 min. At this point, all the solid haddissolved. GC analysis indicated that the reaction was complete. Thebatch was cooled to ambient temperature and concentrated under reducedpressure to give 3-bromo-2-methylbenzoyl chloride as a light brown oil.This material was dissolved in anhydrous CH₂Cl₂ (350 mL) and transferredto an addition funnel. This solution was added drop wise to a solutionof 2-amino-2-methyl-1-propanol (182.4 g, 2.05 mol) in anhydrous CH₂Cl₂(1.0 L) over 45 min. During the addition, the internal temperature waskept between 10° C. to 20° C. by cooling with an ice-water bath. Theaddition funnel was rinsed with anhydrous CH₂Cl₂ (50 mL). The batch wasstirred at ambient temperature for 3 h. GC analysis indicated that thereaction was complete. The salts were filtered-off and the filter cakewas rinsed with CH₂Cl₂ (1 L). The filtrate was washed with sat. aqNaHCO₃ (500 mL, prepared from 200 g NaHCO₃ and 1.6 L tap water) andbrine (500 mL, prepared from 750 g NaCl and 1.5 L tap water). Theseparated organic layer was dried over Na₂SO₄ (400 g) and filtered.Material transfer and filter cake rinsing was completed with CH₂Cl₂ (1L). The filtrate was concentrated under reduced pressure. To theresulting residue was slowly added thionyl chloride (302 mL, 4.16 mol).Exothermic reaction occurred. The reactor was attached to aqueous NaOHsolution scrubber (the amount of NaOH was not listed above) and thebatch was stirred for 2 h upon completion of the addition. GC analysisindicated that the reaction was complete. Thionyl chloride was removedunder reduced pressure. The residue was cooled with an ice-water bath.MeOH (100 mL) was slowly added (very exothermic), followed by 20 wt %NaOH solution (600 mL, prepared from 200 g NaOH and 0.8 L tap water).The pH of the reaction mixture was determined to be 14. Tap water (1 L)was added, and the batch was stirred until most of the solids dissolved.The mixture was extracted with MTBE (800 mL×3). The organic layers werecombined, washed with brine (500 mL), dried over Na₂SO₄ (322 g), andconcentrated under reduced pressure. The residue was purified by passagethrough a silica-gel plug (1 kg silica gel in a 3 L filter funnel loadedwith 2 L of hexane) eluted with 15% EtOAc/hexane (7.5 L). The first 5.5L of eluent contained 2. Concentration under reduced pressure afforded217 g (87%) of 2 as an orange oil. HPLC indicated its purity was 96.5 A%. LCMS (m/z) 268.14 (M⁺+1, ⁷⁹Br), 270.03 (M⁺+1, ⁸¹Br). ¹H NMR (400 MHz,CDCl₃) δ 1.40 (6H, s), 2.61 (3H, s), 4.09 (2H, s), 7.06 (1H, dd, J=8.4,7.2 Hz), 7.60 (1H, d, J=7.2 Hz), 7.62 (1H, dd, J=8.4, 1.2 Hz). ¹³C NMR(100 MHz, CDCl₃) δ 20.9, 28.5 (2C), 68.2, 79.0, 126.7, 126.8, 129.0,130.3, 134.7, 137.9, 162.3.

Procedure:

A 3 L, four-necked flask was equipped with a magnetic stir bar, athermocouple, an addition funnel, and a reflux condenser with N₂ inlet.To it were charged with Mg turnings (4.76 g, 0.196 mol) and anhydrousTHF (20 mL). Next, 1,2-dibromoethane (0.5 mL) was added and vigorousstirring was performed for a few minutes. Activation of the Mg becameevident (bubbling, blackening). The batch was heated to 55° C., and asolution of 2 (50 g, 0.186 mol) in anhydrous THF (480 mL) was added over10 min. An exothermic reaction started and it persisted for 10 min. Mostof the Mg had dissolved at this point and the mixture had turned to agreenish suspension. Heating at reflux was continued for 30 min. Asolution of 4(5)-imidazolecarboxaldehyde (8.94 g, 0.093 mol) dissolvedin anhydrous THF (250 mL) was added all at once; the mixture wasrefluxed overnight. After cooled to ambient temperature, the mixture wascooled with ice-water bath. Saturated aq NH₄Cl (500 mL, prepared from600 g of NH₄Cl and 1.65 L of tap water) was added, keeping the internaltemperature below 24° C. After addition was completed, cooling wasremoved and the mixture was stirred at ambient temperature for 40 min.The layers were separated; the organic layer was washed with saturatedaq NH₄Cl (250 mL), dried over anhydrous MgSO₄ (10 g), filtered, andconcentrated under reduced pressure. The resulting oily solid (75.1 g)was swished with 10% EtOAc-hexane (300 mL) for 1 h. The retained aqueouslayer was extracted with EtOAc (500 mL), and the organic layer driedover MgSO₄ (5 g), filtered, and concentrated under reduced pressure.Residual solids from the second extract (3.3 g) were swished with 10%EtOAc-hexane (100 mL) for 1 h. Both batches of swished material werefiltered and combined, then rinsed with 10% EtOAc-hexane (100 mL). Thecombined solid was dried under high vacuum to give 19.1 g (72% based on4(5)-imidazolecarboxaldehyde) of an off-white powder. HPLC analysisindicated it to be a ˜4:1 mixture of 3 and 4. LCMS (m/z) 286.14 (M⁺+1).¹H NMR (400 MHz, CD₃OD) δ 1.38 (6H, s), 2.33 (3H, s), 4.17 (2H, s), 6.05(1H, s), 6.65 (1H, s), 7.30 (1H, t, J=7.6 Hz), 7.46 (1H, dd, J=8.0, 1.2Hz), 7.63 (1H, s), 7.73 (1H, d, J=7.6 Hz). ¹³C NMR (100 MHz, CD₃OD) δ16.2, 28.6 (2C), 67.8, 68.7, 80.5, 118.7, 126.8, 129.7, 130.0, 130.2,136.1, 136.8, 141.4, 144.0, 166.6.

Procedure:

A 5 L, three-necked flask was equipped with a stir bar, a refluxcondenser with N₂ inlet, a thermocouple, and a stopper. To it wascharged 3 (81.0 g, 0.28 mol, ˜4:1 mix with 4), MnO₂ (243.4 g, 2.80 mol),and 1,2-dichloroethane (2.6 L). The mixture was stirred at 75° C. for 1h. At this point, HPLC analysis indicated that the reaction wascomplete. Heating was halted and the mixture cooled to 60° C. The batchwas filtered through a Celite pad (500 g, packed with 1 L of1,2-dichloroethane), and the filter cake rinsed with THF (6 L). Thefiltrate and wash were concentrated to a small volume (˜350 mL), andhexane (3 L) was added in a thin stream with stirring. After stirringfor 15 min the solids were collected, the filter cake was rinsed withhexane (200 mL), then it was dried under reduced pressure at 60° C. for6 h. Thus was obtained 68.0 g (86%) of 4 as a powder. ¹H NMR analysisindicated it the presence of ˜5 mol % THF. LCMS (m/z) 284.26 (M⁺+1). ¹HNMR (400 MHz, CD₃OD) δ 1.39 (6H, s), 2.41 (3H, s), 4.20 (2H, s), 7.37(1H, t, J=7.6 Hz), 7.48 (1H, s), 7.53 (1H, d, J=7.6 Hz), 7.72 (1H, d,J=7.6 Hz), 7.93 (1H, s). ¹³C NMR (100 MHz, CD₃OD) δ 17.8, 28.6 (2C),68.9, 80.6, 126.7, 130.9, 131.2, 132.5 (2C), 136.9, 138.6, 140.6, 142.0,165.6, 190.5.

Procedure:

A three-necked, 5 L flask equipped with a magnetic stir bar, refluxcondenser, N₂ inlet, and addition funnel was charged with 4 (139.0 g,0.49 mol) and THF (3.2 L). To the stirred solution was added MeMgCl (22wt % in THF, 542.0 g), keeping the internal temperature below 40° C.When addition was complete, the batch was heated at 65° C. for 3 h andeventually became dark brown. At this point, TLC analysis showed thereaction to be complete. The batch was cooled with ice-water bath to 10°C. The reaction was quenched by drop wise addition of saturated aq NH₄Cl(1.0 L, prepared from 835 g of NH₄Cl and 2.5 L of tap water) to themixture while keeping the internal temperature below 20° C. After thisaddition was complete, the layers were separated. The organic layer waswashed with a 1:2 mixture of tap water (500 mL) and brine (1000 mL,prepared from 1 kg of NaCl and 2 L of tap water), and the layers wereseparated. The organic layer was washed with brine (300 mL, as above),separated, and dried over MgSO₄ (100 g). After filtration, it wasconcentrated under reduced pressure. The residue was dried under thehigh vacuum to give 162.8 g of 5 as a brown foam. ¹H NMR analysisindicated that the material contaminated some EtOAc and THF but was pureenough to carry on to the next step without further purification. LCMS(m/z) 300.22 (M⁺+1). ¹H NMR (400 MHz, CD₃OD) δ 1.36 (6H, s), 1.92 (3H,s), 2.16 (3H, s), 4.15 (2H, s), 6.82 (1H, s), 7.26 (1H, dd, J=8.0, 7.2Hz), 7.38 (1H, d, J=7.2 Hz), 7.57 (1H, s), 7.86 (1H, d, J=8.0 Hz). ¹³CNMR (100 MHz, CD₃OD) δ 18.3, 28.6 (2C), 29.8, 68.6, 73.4, 80.5, 118.3,126.4, 129.6, 129.8, 131.9, 136.2, 137.0, 145.1, 147.2, 167.1.

Procedure:

A 2 L heavy-walled pressure bottle was charged with 5 (crude, 162.8 g),10 wt % Pd/C (25.0 g), a magnetic stir bar, and aqueous 4 N HCl solution(1.35 L, prepared from 500 mL of 12 N HCl and 1 L of tap water). It wassealed, purged sequentially with N₂, and H₂ to 55 psi, and stirredvigorously in an 85° C. oil bath for 21 h. At this point, ¹H NMR assayindicated that the reaction was complete. The batch was filtered througha Celite pad (150 g, packed with 200 mL of HPLC grade water), and thefilter cake rinsed with 50% MeOH-HPLC grade water (1 L). Filtrates wereconcentrated under reduced pressure, and the wet residue wasco-evaporated with dry MeOH (500 mL×3). The batch was dissolved in MeOH(2.0 L), 4 M HCl in dioxane (1 L) was added, and the mixture was heatedat 65° C. for 16 h. At this point, HPLC analysis indicated that thereaction was complete. Heating was halted and the batch concentratedunder reduced pressure. The resulting thick syrup/solid was cooled in anice-water bath and treated with saturated aqueous NaHCO₃ (1.8 L,prepared from 450 g of NaHCO₃ dissolved in 3.1 L of tap water). The pHof the mixture was found to be 8-9. Tap water (500 mL) was added to themixture, which was extracted with EtOAc (2 L×4). The separated organiclayers were combined and divided into 2 portions for ease of handling.Each was washed with brine (500 mL, prepared from 1.0 kg of NaCldissolved in 2 L of tap water). The combined organic phases were driedover anhydrous MgSO₄ (200 g), filtered, and concentrated under reducedpressure to afford a wet solid. This material was swished in 50%EtOAc-hexane (2.0 L) for 16 h. The slurry was filtered and the filtercake rinsed with 10% EtOAc-hexane (500 mL). Drying under high vacuum for16 h gave 94.8 g (79%) of 6 as a white powder. This filtrate wasconcentrated under reduced pressure and the residue was swished with 10%EtOAc-hexane (300 mL) for 3 h. Solids were collected on a filter and thefilter cake was rinsed with 10% EtOAc-hexane (40 mL). Drying under highvacuum afforded an additional 9.4 g (8%) of 6 as a yellowish powder.LCMS (m/z) 245.16 (M⁺+1). ¹H NMR (400 MHz, CD₃OD) δ 1.55 (3H, d, J=6.8Hz), 2.50 (3H, s), 3.87 (3H, s), 4.46 (1H, q, J=6.8 Hz), 6.74 (1H, t,J=1.2 Hz), 7.17 (1H, dd, J=8.0, 7.6 Hz), 7.26 (1H, dd, J=8.0, 1.2 Hz),7.54 (1H, dd, J=7.6, 1.2 Hz), 7.57 (1H, d, J=1.2 Hz). ¹³C NMR (100 MHz,CD₃OD) δ 16.2, 21.3, 35.3, 52.6, 117.8, 126.8, 128.9, 131.4, 133.3,136.4, 137.2, 143.0, 146.5, 171.1.

Procedure:

To a 250 mL single-necked flask equipped with a magnetic stirring barand rubber septum containing a stirred suspension of 6 (1.0 g, 4.09mmol) in THF (25 mL) under N₂ was added LiBH₄ (2.0 M in THF, 6.1 mL,12.27 mmol) in one portion. To the resulting clear solution was addedMeOH (1.0 mL then 2×0.5 mL) over 1 h. After 2 h, HPLC analysis indicatedreaction had proceeded 22%. Additional LiBH₄ (2.0 M in THF, 6.1 mL,12.27 mmol) and MeOH (2.0 mL) were added to the mixture in like fashion,and the batch was stirred for 16 h. At this time, analysis by HPLCindicated 96% conversion. Again, LiBH₄ (2.0 M in THF, 6.1 mL, 12.27mmol) and MeOH (2.0 mL) were added and the mixture stirred for 2 h. Thistime, HPLC analysis indicated the reaction to be complete. Neat acetone(5.0 mL) was added drop wise to the mixture, resulting in an exothermicreaction in which the batch temperature rose to 38° C. The mixture wascooled with an ice-water bath. When cool, a 4 N aqueous HCl solution(5.5 mL, prepared from 100 mL of 12 N HCl and 200 mL of tap water) wasadded carefully to adjust the pH to 7. The batch was diluted with tapwater (25 mL) and the resulting cloudy solution was extracted with EtOAc(3×100 mL). HPLC assay indicated that only the first two extractscontained product. All organic extracts were combined and dried overanhydrous MgSO₄ (16.2 g). The batch was filtered through a filter paperand the filter cake rinsed with EtOAc (100 mL). Filtrate and rinses wereconcentrated under reduced pressure. The residue was swished with EtOAc(50 mL) for 10 min while cooling with an ice-water bath, and then thesolids were collected on a filter. The filter cake was rinsed with coldEtOAc (20 mL) and dried under high vacuum (1.18 g). The ¹H NMR spectrumof this material suggested it to be an HCl salt; HPLC analysis showedone peak. The powder was suspended in tap water (30 mL) and 20 wt %aqueous NaOH solution (0.69 g); the pH of the mixture was 9. Aftervigorously stirring the mixture for 3 h, the batch was filtered and thefilter cake was rinsed with MTBE (25 mL). Analysis by ¹H NMR of thispowder indicated it to be free base. The powder was dried in a vacuumover at 60° C. for 5 h to give 730 mg (82%) of compound according toFormula I as a white powder. mp=189-195° C. LCMS (m/z) 217.12 (M⁺+1). ¹HNMR (400 MHz, DMSO-d₆) δ 1.45 (3H, d, J=6.8 Hz), 2.24 (3H, s), 4.31 (1H,q, J=6.8 Hz), 4.49 (2H, d, J=3.6 Hz), 5.02 (1H, br s), 6.69 (1H, s),7.00 (1H, br d, J=7.6 Hz), 7.06 (1H, t, J=7.6 Hz), 7.17 (1H, br d, J=7.6Hz), 7.48 (1H, s), 11.75 (1H, br s). ¹³C NMR (100 MHz, DMSO-d₆) δ 13.4,21.0, 33.3, 61.9, 116.5, 124.8, 125.1, 125.3, 132.6, 134.7, 140.1,141.5, 144.0.

The synthesis may be summarized as follows:

Methods of Use

The compounds of the invention may be used to treat chronic pain. To“treat,” as used here, means to deal with medically. It includesadministering a compound of the invention to prevent pain as well as toalleviate its severity.

Pain is of two types: chronic and acute. An “acute pain” is a pain ofshort duration having a sudden onset. One type of acute pain, forexample, is cutaneous pain felt on injury to the skin or othersuperficial tissues, such as caused by a cut or a burn. Cutaneousnociceptors terminate just below the skin, and due to the highconcentration of nerve endings, produce a well-defined, localized painof short duration. “Chronic pain” is a pain other than an acute pain.

The compounds of the invention may be used to treat chronic pain.Chronic pain includes neuropathic pain, inflammatory pain, headachepain, somatic pain, visceral pain and referred pain.

I. Neuropathic Pain

The compounds of the invention may be used to treat neuropathic pain.Neuropathic pain includes pain associated with neuralgia,deafferentation, complex regional pain syndromes, and neuropathy.

1. Neuralgia

Neuralgia is a pain that radiates along the course of one or morespecific nerves usually without any demonstrable pathological change inthe nerve structure. The causes of neuralgia are varied. Chemicalirritation, inflammation, trauma (including surgery), compression bynearby structures (for instance, tumors), and infections may all lead toneuralgia. In many cases, however, the cause is unknown orunidentifiable. Neuralgia is most common in elderly persons, but it mayoccur at any age. Neuralgia includes, for example, trigeminal neuralgia,spinal stenosis, postherpetic neuralgia, glossopharyngeal neuralgia,pain associated with nerve entrapment disorders, sciatica, and atypicalfacial pain.

2. Deafferentation

Deafferentation indicates a loss of the sensory input from a portion ofthe body, and can be caused by interruption of either peripheral sensoryfibres or nerves from the central nervous system. Deafferentation painsyndrome includes, for example, post-stroke pain, phantom pain, andparaplegia.

3. Complex Regional Pain Syndromes (CRPSs)

CRPS is a chronic pain syndrome with two forms. CRPS 1 currentlyreplaces the term “reflex sympathetic dystrophy syndrome.” It is achronic nerve disorder that occurs most often in the arms or legs aftera minor or major injury. CRPS 1 is associated with severe pain; changesin the nails, bone, and skin; and an increased sensitivity to touch inthe affected limb. CRPS 2 replaces the term causalgia, and results froman identified injury to the nerve.

4. Neuropathy

Neuropathy is a functional or pathological change in a nerve and ischaracterized clinically by sensory or motor neuron abnormalities.Central neuropathy is a functional or pathological change in the centralnervous system.

Peripheral neuropathy is a functional or pathological change in one ormore peripheral nerves. Either condition can lead to pain which may betreated by the compounds of the invention.

Some causes of neuropathy include heredity disorders, such asCharcot-Marie-Tooth disease, Friedreich's ataxia; systemic or metabolicconditions, such as diabetes, dietary deficiencies (especially vitaminB-12 deficiency), excessive alcohol use, uremia, and cancer; andinfectious conditions, such as AIDS, hepatitis, and diphtheria; exposureto toxic compounds, such as the solvents used in industrial processes,heavy metals (lead, arsenic, mercury, etc.); and chemotherapy.

Neuropathy may involve a function or pathological change to a singlenerve or nerve group (monneuropathy) or a function or pathologicalchange affecting multiple nerves (polyneuropathy). Other types ofneuropathy includes generalized peripheral neuropathies, distalaxonopathies, myelinopathies, neuronopathies, and focal entrapmentneuropathies, all of which can lead to chronic pain which may be treatedby the compounds of the invention.

II. Inflammatory Pain

The compounds of the invention may be used to treat chronic painassociated with any of the following conditions: arthritis, such asrheumatoid arthritis, juvenile rheumatoid arthritis, systemic lupuserythematosus (SLE), gouty arthritis, scleroderma, osteoarthritis,psoriatic arthritis, and ankylosing spondylitis; connective tissuedisorders, such as spondyloarthritis and dermatomyositis; injury, suchas stretching of a tissue or joint, that results in chronic inflammatorypain; infection; neuritis, such as brachial neuritis and retrobulbarneuropathy; vestibular neuritis; and inflammation of the joints, such asthat caused by bursitis or tendonitis.

III. Headache Pain

The compounds of the invention may be used to treat chronic painassociated with any of the following headache conditions. A headache(cephalgia) is a condition of mild to severe pain in the head. It mayindicate an underlying local or systemic disease or be a disorder initself. Examples of headaches include muscular/myogenic headache, suchas tension headache; vascular headache, such as migraine, clusterheadaches, and headaches resulting from high blood pressure; tractionand inflammatory headaches resulting from other disorders such as strokeor sinus infection; hormone headache; rebound headache (medicationoveruse headaches); chronic sinusitis headache; organic headache; andictal headaches.

IV. Somatic Pain

The compounds of the invention may be used to treat chronic painassociated with any of the following somatic pain conditions: excessivemuscle tension, such as that caused by sprains or strains; repetitivemotion disorders, such as those resulting from overuse of the hands,wrists, elbows, and shoulder; muscle disorders such as polymyositis,dermatomyositis, lupus, fibromyalgia, myalgia, polymyalgia rheumatica,macrophagic myofasciitis, and rhabdomyolysis; and muscle pain secondaryto neurological and neuromuscular disorders;

V. Visceral Pain

The compounds of the invention may be used to treat chronic painassociated with any of the following visceral pain conditions. Visceralpain originates from body's viscera, or organs. Examples of visceralpain include the following: functional visceral pain, such as painassociated with functional irritable bowel syndrome, functionalabdominal pain, functional constipation, functional dyspepsia,functional gastroesophageal reflux disease, and non-cardiac chest pain;pain associated with chronic gastrointestinal inflammation, such asgastritis, inflammatory bowel disease, Crohn's disease, ulcerativecolitis, diverticulitis and gastroenteritis; pain associated withinterstitial cystitis, urinary tract infections, pancreatitis andhernia.

VI. Allodynia

The A afferent fibers (A-beta and A-delta fibers) can be stimulated at alower threshold than C fibers, and appear to be involved in thesensation of chronic pain. Under normal conditions, low thresholdstimulation of these fibers (such as a light brush or tickling) is notpainful. Under certain conditions such as those following nerve injuryor in the herpes virus-mediated condition known as shingles theapplication of even such a light touch or the brush of clothing can bevery painful. This condition is termed allodynia and appears to bemediated at least in part by A-beta afferent nerves. C fibers may alsobe involved in the sensation of chronic pain, but if so it appears clearthat persistent firing of the neurons over time brings about some sortof change which now results in the sensation of chronic pain. Thechronic pain associated with allodynia may be treated with the compoundsof the invention.

Formulation and Administration

Compounds of the invention are formulated as pharmaceuticalcompositions. “Pharmaceutical composition,” as used here, means acomposition that is suitable for administering to human patients for thetreatment of disease. In one embodiment, therefore, the compounds of theinvention are formulated as pharmaceutically acceptable salts andfurther include one or more pharmaceutically acceptable excipients.

A “pharmaceutically acceptable salt” is any salt that retains theactivity of the parent compound and does not impart any additionaldeleterious or untoward effects on the subject to which it isadministered and in the context in which it is administered compared tothe parent compound. A pharmaceutically acceptable salt also refers toany salt which may form in vivo as a result of administration of anacid, another salt, or a prodrug which is converted into an acid orsalt. Pharmaceutically acceptable salts of acidic functional groups maybe derived from organic or inorganic bases. The salt may comprise a monoor polyvalent ion. Of particular interest are the inorganic ionslithium, sodium, potassium, calcium, and magnesium. Organic salts may bemade with amines, particularly ammonium salts such as mono-, di- andtrialkyl amines or ethanol amines. Salts may also be formed withcaffeine, tromethamine and similar molecules. Hydrochloric acid or someother pharmaceutically acceptable acid may form a salt with a compoundthat includes a basic group, such as an amine or a pyridine ring.

Compounds of the invention may also be formulated as prodrugs. A“prodrug” is a compound which is converted to a therapeutically activecompound after administration, and the term should be interpreted asbroadly herein as is generally understood in the art. While notintending to limit the scope of the invention, conversion may occur byhydrolysis of an ester group or some other biologically labile group.Generally, but not necessarily, a prodrug is inactive or less activethan the therapeutically active compound to which it is converted. Esterprodrugs of the compounds disclosed herein are specificallycontemplated. While not intending to be limiting, an ester may be analkyl ester, an aryl ester, or a heteroaryl ester. The term alkyl hasthe meaning generally understood by those skilled in the art and refersto linear, branched, or cyclic alkyl moieties. C₁₋₆ alkyl esters areparticularly useful, where alkyl part of the ester has from 1 to 6carbon atoms and includes, but is not limited to, methyl, ethyl, propyl,isopropyl, n-butyl, sec-butyl, iso-butyl, t-butyl, pentyl isomers, hexylisomers, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, andcombinations thereof having from 1-6 carbon atoms, etc.

Compounds of the invention may be administered orally, transdermally,topically, intraperitoneally, parenterally, subcutaneously,intranasally, intrathecally, intramuscularly, intravenously andintrarectally.

“Pharmaceutically acceptable excipients” refers to those ingredientscomprising the vehicle in which the compounds of the invention areadministered. Excipients are usually inert. Their selection depends onhow the drug is to be administered. Compounds of the invention may beconfected as a powder, pill, tablet or the like, or as a solution,emulsion, suspension, aerosol, syrup or elixir suitable for oral orparenteral administration or inhalation.

For solid dosage forms or medicaments, non-toxic solid carriers include,but are not limited to, pharmaceutical grades of mannitol, lactose,starch, magnesium stearate, sodium saccharin, the polyalkylene glycols,talcum, cellulose, glucose, sucrose and magnesium carbonate. The soliddosage forms may be uncoated or they may be coated by known techniquesto delay disintegration and absorption in the gastrointestinal tract andthereby provide a sustained action over a longer period. For example, atime delay material such as glyceryl monostearate or glyceryl distearatemay be employed. They may also be coated by the technique described inU.S. Pat. No. 4,256,108, No. 4,166,452, and No. 4,265,874, the contentsof which are incorporated herein by reference, to form osmotictherapeutic tablets for control release.

Liquid pharmaceutically administrable dosage forms can, for example,comprise a solution or suspension of one or more of the presently usefulcompounds and optional pharmaceutical adjutants in a carrier, such asfor example, water, saline, aqueous dextrose, glycerol, ethanol and thelike, to thereby form a solution or suspension. If desired, thepharmaceutical composition to be administered may also contain minoramounts of nontoxic auxiliary substances such as wetting or emulsifyingagents, pH buffering agents and the like. Typical examples of suchauxiliary agents are sodium acetate, sorbitan monolaurate,triethanolamine, sodium acetate, triethanolamine oleate, etc. Actualmethods of preparing such dosage forms are known, or will be apparent,to those skilled in this art; for example, see Remington'sPharmaceutical Sciences, Mack Publishing Company, Easton, Pa., 16thEdition, 1980. The composition of the formulation to be administered, inany event, contains a quantity of one or more of the presently usefulcompounds in an amount effective to provide the desired therapeuticeffect.

The compounds of the invention may be administered at pharmaceuticallyeffective doses. Such dosages are usually the minimum dose necessary toachieve the desired therapeutic effect; in the treatment of chronicpain, this amount would be roughly that necessary to reduce thediscomfort caused by the pain to tolerable levels. For human adults suchdoses generally will be in the range of about 0.01-50 mg/kg/day, andoften in the range of 0.05-25 mg/kg/day. However, the actual amount ofthe compound to be administered in any given case will be determined bya physician taking into account the relevant circumstances, such as theseverity of the pain, the age and weight of the patient, the patient'sgeneral physical condition, the cause of the pain, and the route ofadministration.

The most effective pain-relieving drugs are also the most heavilysedating. The compounds of the invention, in contrast, can effectivelyrelieve pain at doses that are non-sedating or only minimally sedating.“Sedating,” as used here, means causing that level of sedation whichwould be described by a score of three or greater on the StanfordSleepiness Scale. “Non-sedating” or “minimally sedating” means causingno more sedation than would be described by a score of 3 or less on theStanford Sleepiness Scale. In this widely used measurement of alertness,a patient sits quietly with his eyes closed for one minute and thendescribes his current state of alertness using one of eightdescriptions, as set forth below:

TABLE 1 Stanford Sleepiness Scale SCORE DESCRIPTION 1 Feeling active,vital, alert, or wide awake 2 Functioning at high levels, but not atpeak; able to concentrate 3 Awake, but relaxed; responsive but not fullyalert 4 Somewhat foggy, let down 5 Foggy; losing interest in remainingawake; slowed down 6 Sleepy, woozy, fighting sleep; prefer to lie down 7No longer fighting sleep, sleep onset soon; having dream-like thoughts XAsleep

In one embodiment, Compound I (or one or both of its enantiomers) isadministered to a patient to treat chronic pain at a dose that resultsin a sleepiness score of no more than 1 on the Stanford SleepinessScale. In another embodiment, Compound I (or one or both of itsenantiomers) is administered to a patient to treat chronic pain at adose that results in a sleepiness score of no more than 2 on theStanford Sleepiness Scale. In another embodiment, Compound I (or one orboth of its enantiomers) is administered to a patient to treat chronicpain at a dose that results in a sleepiness score of no more than 3 onthe Stanford Sleepiness Scale.

At high doses the compounds of the invention are sedating. Hence, inanother embodiment, Compound I (or one or both of its enantiomers) isadministered to a patient to treat chronic pain at a dose that resultsin a sleepiness score of four or greater on the Stanford SleepinessScale.

The invention is further illustrated below using the following Examples.

EXAMPLES Example 1 Analgesia Vs. Sedation

Compound I (racemic mixture) was assayed for its ability to alleviatesulprostone-induced tactile hypersensitivity and for sedating activityat various doses. The tactile hypersensitivity of 5-6 mice (weighingapprox. 25 g) per group was scored every five minutes between 15 and 50minutes following intraperitoneal dosing with 300 ng/kg (in a 1 mL/kgvolume) sulprostone (Cayman Chemical Company, Ann Arbor, Mich.).Compound I or vehicle was administered intraperitoneally 15 minutesprior to sulprostone.

Tactile hypersensitivity was quantified by scoring the response of miceto light stroking of their flanks with a small paintbrush, which is notnormally painful. The mice were rated on the following scale: a score of“2” was given to animals showing aggressive escape responses along withsqueaking and biting at the brush; a score of “1” was given to animalsexhibiting mild squeaking with attempts to escape; and a score of “0”was given if the animal showed no response to the light stroking of thepaintbrush. The scores were summed to generate a cumulative score of 0to 16 as described in Minami et al., Pain 57:217-223 (1994).Sulprostone-treated mice typically have a score of 14 andvehicle-treated mice typically have a score of 5-6. Chronic paincompounds including gabapentin and amitriptyline are active in thismodel consistent with effects in other chronic pain models and chronicpain patients, as described in Gil D W, Cheevers C V and Donello J E, BrJ Pharmacol 153: 769-74 (2008).

In addition, the locomotor activity of 5-6 mice per group was measuredin a five minute period 30 minutes following intraperitoneal dosing withcompound I. The activity is determined automatically by placing the micein an exploratory chamber (Omnitech Electronic) that has photocell beamscriss-crossing the box that are interrupted as the animal moves around.The computer analyses this movement and records total activity.Locomotor activity relative to vehicle-treated animals was expressed asa percentage; percentage sedation was calculated as 100% minus thepercent locomotor activity. Statistical calculations of significance forin vivo studies were done using a two-tailed Student's t-test.

As shown in Figures 1 and 2, Compound I administered at a dose of 1mg/kg was minimally sedating and at a dose of 3 mg/kg was highlysedating. This was 10 fold-greater than the dose (0.3 mg/kg) thatproduced a reduction in sulprostone induced hypersensitivity back tocontrol levels. Thus, compound I reduces pain at doses that are notsedating.

Example 2 Analgesia Vs. Sedation

Studies with Compound I and dexmedetomidine show that there isa >10-fold margin of safety for pain reduction vs. sedation withCompound I (Figure 3), but no margin for dexmedetomidine (Figure 4).They further show that Compound I is advantageous as an agent forrelieving pain and can be taken systemically without the need forintravenous delivery.

Example 3 Activity in a FLIPR Assay

Compounds of the invention were evaluated in a FLIPR assay in order todetermine their specificity for five alpha adrenergic receptors. TheFLIPR assay measures release of intracellular calcium in response toreceptor activation. Cells are preloaded with a calcium sensitive dyethat fluoresces in response to calcium. Receptors that activate the Gprotein, Gq, elicit the response. Alpha adrenergic receptors, whichnormally couple to Gi, trigger the increase in intracellular calciumwhen coexpressed with a hybrid Gq protein containing a Gi receptorrecognition domain, designated Gq/i5 (Conklin et al., Nature 363:274-6(1993)) or with the promiscuous G protein G_(a16).

The assay was performed essentially as follows. HEK 293 cells stablyexpressing an alpha-2 receptor and either the chimeric G-protein Gqi5 orthe promiscuous G protein G16 were plated in poly-D-lysine coated384-well plates at 20,000-25,000 cells per well and grown overnight inDMEM supplemented with 10% FBS. Cells were washed twice with HBSS/HEPESBuffer (1× Hanks Buffered Salt Solution, 20 mM HEPES, pH 7.4) prior tothe addition of Fluo-4-AM (4 μM Fluo-4-AM, 0.04% pluronic acid inHBSS/HEPES Buffer), a calcium-sensitive dye. Cells were loaded with dyefor 60 minutes at 37° C., then washed 4 times with HBSS/HEPES Buffer.The reaction was initiated by the addition of the appropriate dilutionsof compounds (final concentration between 0.64 nM-10,000 nM) and thetransient calcium signal captured. The peak height of the calcium curvewas determined and utilized for calculation of EC₅₀ and efficacy usingActivityBase.

Table 2, below, shows agonist and antagonist activity at the alpha-1A,alpha-1B, alpha-2A, alpha-2B, and alpha-2C adrenergic receptors. Data inthe first line in each column shows agonism; the first number is theEC₅₀, with the relative efficacy as compared to norepinephrine inparenthesis. Data in the second line, when present, shows antagonism;the first number is the IC₅₀, with the percent antagonism inparenthesis. As shown in Table 2, the (R) enantiomer of Compound I is analpha-2C and alpha-2B agonist, and an alpha-2A antagonist. The (S)enantiomer is an alpha-2 pan agonist having some alpha-1A agonistactivity. The racemate (Compound I) is an alpha-2 pan agonist havingsome alpha-1A agonist activity.

TABLE 2 Activity in a FLIPR assay Compound α-1A α-1B α-2A α-2B α-2CCompound I 1632 (.69) >10000 53.7 (.75) 5.5 (.94)  15.7 (.98)   >8300Compound II >10000 >10000 >10000 52.8 (.68) 171 (.64) nd (39%)   >8300144 (96%)  Compound III 445 (.76)  >8300 72 (.9) 3 (.93) 8 (1)

What is claimed is:
 1. A pharmaceutical composition comprising an esterprodrug of a compound of the formula:

wherein the ester is an alkyl ester, an aryl ester or a heteroarylester.
 2. The pharmaceutical composition of claim 1, wherein thecomposition comprises an ester prodrug of a compound of the formula:

wherein the ester is an alkyl ester, an aryl ester or a heteroarylester.
 3. The pharmaceutical composition of claim 1, wherein thecomposition comprises a compound represented by

wherein the ester is an alkyl ester, an aryl ester or a heteroarylester.
 4. The pharmaceutical composition of claim 1, wherein the esteris an alkyl ester.
 5. The pharmaceutical composition of claim 2, whereinthe ester is an alkyl ester.
 6. The pharmaceutical composition of claim3, wherein the ester is an alkyl ester.
 7. The pharmaceuticalcomposition of claim 1, wherein the ester is a C₁₋₆ alkyl ester.
 8. Thepharmaceutical composition of claim 2, wherein the ester is a C₁₋₆ alkylester.
 9. The pharmaceutical composition of claim 3, wherein the esteris a C₁₋₆ alkyl ester.
 10. The pharmaceutical composition of claim 7,wherein the alkyl part of the ester is selected from the groupconsisting of methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl,iso-butyl, t-butyl, pentyl isomers, hexyl isomers, cyclopropyl,cyclobutyl, cyclopentyl and cyclohexyl.
 11. The pharmaceuticalcomposition of claim 8, wherein the alkyl part of the ester is selectedfrom the group consisting of methyl, ethyl, propyl, isopropyl, n-butyl,sec-butyl, iso-butyl, t-butyl, pentyl isomers, hexyl isomers,cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl.
 12. Thepharmaceutical composition of claim 9, wherein the alkyl part of theester is selected from the group consisting of methyl, ethyl, propyl,isopropyl, n-butyl, sec-butyl, iso-butyl, t-butyl, pentyl isomers, hexylisomers, cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl.